This invention relates to feedback control loop systems. More particularly, it relates to feedback control loop systems for controlling elements which have a variable gain and frequency response and which produce an output signal proportional to the element gain and its frequency response.
In most feedback control loop systems, the basic problem to be solved is simply that of generating and feeding back an error correction signal whose parameters are essentially independent of any effects of gain or frequency response by the element to be controlled. In such a situation the gain and frequency response to be applied to the error correction signal remains at all times substantially constant, regardless of the output of the controlled element, since the controlled element has no impact upon the bandwidth of the feedback control loop. Conversely, in a conventional automatic gain control circuit the problem addressed is that of amplifying or attenuating a signal utilized in an open-loop control system. In that situation the applied gain is utilized for stabilization of another parameter of operation of the controlled element which does not itself affect the applied gain or frequency response.
As an example of servo control systems illustrating the closest-known current understading of the adaptive control problem, reference is made to an article by Richard L. Melton of Industrial Drives, A Kollmorgen Company, entitled "Real-Time Adaptive Velocity Control," appearing in Powerconversion & Intelligent Motion, Volume 12, No. 5, May 1986, page 40-44. This article is based in part on the Technical Report TR-85-CO2, "Real-Time Adaptive Control," March 1985, by Wayne T. Culberson, Research Scientist, Kollmorgen Corporation, Industrial Drives Division. The article teaches of the need for optimum servo compensation over a wide range of load parameter variations. However, the article fails to recognize the existence or possibility of a critical low-frequency pole shift in the disclosed design which moves concurrently with a zero of interest. Moreover, a critical analysis reveals that hardware disclosed in Appendix C and Appendix D of the article do not correspond to the block diagram shown in FIG. 3 of the article. Furthermore, the block diagram of FIG. 3 cannot be realized in practice because of the missing pole at the block labeled KI/S. Consequently, the teachings of the article are inadequate for a solution of the problem addressed.
Heretofore, except for U.S. Pat. No. 4,092,530 to the present inventor, it has not been deemed necessary to address the compound problem of a second-order stabilization in which it is necessary to stabilize the loop gain and frequency response of the system stabilizing feedback loop, wherein the controlled element itself causes variations in the loop gain and frequency response and in which the operating parameters of the controlled element thus affect the gain and bandwidth of the feedback control system. In such a compound feedback control system, it is necessary to stabilize simultaneously the feedback loop gain, bandwidth and phase margin to compensate for variations in the gain and frequency response of the controlled element in order to suppress noise and potential system oscillation and stabilize the operating parameters of interest, such as output frequency, output signal level, and feedback control loop bandwidth and phase margin.
One such variable gain and frequency response element for which compounded feedback control is desirable is an electromechanical actuator (such as a DC- or AC-driven motor or thruster) whose higher-frequency gain and frequency response is inverse to loading inertia related directly to the actuator, with or without gearing. Most succinctly, the inertia, or mechanical "pole", P.sub.J, which is inverse to directly relate loading inertia to the actuator, if variable because of loading inertia changes, will cause proportional changes in feedback-controlled loop bandwidth and phase margin in which it is a dominant, or controlling, factor.
To illustrate the stabilization problem more specifically, reference is made to FIG. 1 which shows that the loop-bandwidth affects the changes in loading inertia (and, therefore, pole P.sub.J) related to the actuator have on a typical actuator feedback control loop. The example given illustrates two-to-one variation in loading inertia, as manifest by displacement of the pole P.sub.J. Variation in loading inertia may occur upon the change of a weight of controlled arm of an actuator. This causes a similar two-to-one variation in the loop bandwidth and, therefore, step-response time and following error, or FE, (if rate-stabilized). If this is not compensated for, these changes could be totally unacceptable in the context of overall system performance. But loading inertia and P.sub.J variations of over 100 times-to-nominal occur and should be accommodated from the viewpoint of a system user; this is what has brought about this invention.